The present invention relates to a laserinterferometric process intended for use in etching layers and to a laserinterferometer for the examination of a layer, in particular for carrying out said laserinterferometric process.
Interferometric examinations are often employed in the examination of thin layers. In this application, laserinterferometers have become quite predominant. They are used, e.g. in the etching of thin layers to determine the etch rate and the termination of etching, which will be made more apparent with reference to FIGS. 1 and 2. The layer to be etched 2, which is placed on a substrate 1, is radiated by a laser. With transparent layers, the incident laser beam 10 is partially reflected at the surface of the layer 2 and partially at the interface of layer 2 to substrate 1. The reflected laser beam, thus, is composed of two beams, designated 20 and 22. The phase difference of the two reflected beams depends on the thickness of the layer 2 to be etched. Depending on the phase relation or phase difference, the beams 20 and 22 become stronger or weaker. If the thickness of the layer 2 changes, as is the case in etching, the resulting reflected laser is of changing intensity. A periodicity of the intensity of the reflected laser beam ensues, which is illustrated in FIG. 2. The etch rate can be ascertained from the temporal course of the intensity of the reflected laser beam, i.e. from the period T. When the layer 2 has been etched away, so that the incident beam 10 only strikes the substrate 1, the result is usually a lack of change in the reflectivity, so that the intensity of the reflected laser beam produces a bend in the intensity curve. This bend or stand value indicates the termination point of the etching process, designated 3 in FIG. 2.
Furthermore, deflecting a laser beam by a modulator is known. Such devices, called laser scanners, are employed, by way of illustration, in health care technology or in certain holographic processes.
A disadvantage in the known laserinterferometer devices is that the fixing must be precise in order to analyze specific locations of the layer to be examined.
The object of the present invention is, proceeding from the aforegoing state of the art, to provide a laserinterferometric process and a laserinterferometer, which permits an "on-line" analysis of the surface to be examined, particularly at selected points.
With the present invention, it is possible to obtain a visual representation of the surface to be examined by a laserinterferometer. The laser beam passes over a defined surface of the layer to be examined, whereby it is deflected two-dimensionally. The intensity of the reflected laser beam can be ascertained for the entire examined surface point by point according to the laser passing over point by point or line by line and evaluated via the linked-up data processing system. The invented process simplifies the use of laserinterferometers as no complicated fixing of the location to be analyzed is required prior to the examination, but rather an electronic localization. Thus even very small zones can now be analyzed, e.g. contact holes. Laserinterferometers can now also be employed in cases of a very small ratio of etching surface to masking surface.
In the invented laserinterferometer, it is expedient to employ a HeNe laser or a UV laser. Naturally other types of lasers can also be used. For example in a number of cases, a laser with a wavelength of variable frequency has proven to be suitable.
In the invented process, the laser beam is deflected electro-mechanically, permitting low power input and rapid scanning of the surfaces to be examined.
In an expedient variant of the process, the laser beam is deflected by an electrically modulated crystal.
Optical means are preferably used for the deflection or imaging of the laser beam. In this connection semi-permeable mirrors, by way of illustration, can be employed. It is particularly expedient to direct the laser beam to the layer to be examined via glass fiber optics, whereby the light wave conductor is expediently directed over the surface to be examined. The local resolution power of the laser beam can be increased by a lens system via which the laser beam is visually represented and focused on the layer to be examined.
In order to improve the signal-to-noise ratio, the laser beam is modulated with a frequency that can be fixed and the reflected laser beam at this narrow band frequency is detected.
Another measure is to separate the mixed signals electronically and to suppress the signal noises.
The invented scanning of the layer to be examined permits, on the one hand, an analysis of the entire layer using a data processing system to produce visual representation of the surface of the layer to be examined. On the other hand, the present invention permits making an analysis at any desired point of the layer to be examined, particularly electronically.
Thus, etch rate can be determined for any desired point of the surface of the transparent layer to be etched when etching thin layers.
In another variant of the present process for etching thin layers, the point of termination of any desired point of the surface is detected. Furthermore, the uniformity of the etch rate and the termination point in the layer to be etched can be determined.
In the variant of the process, the selectivity is determined locally resolved on the layer to be etched.
An advantageous method of analyzing the reflected laser beam "on-line" is provided. This method permits rapid adapting of the treatment, particularly of the etching, of the layer to be examined or treated. Moreover the entire surface can be visually represented by timing and examined in connection with the whole. The signals from the data processing system are advantageously employed, to process control, permitting adapting of the etching step directly to the actual condition of the material and especially precise work.
A display screen and/or a plotter is preferably used for the data readout. The intensity of the reflected laser beam can be shown on the display screen, which, thus, delivers a visual representation of the surface to be examined. In order to make this visual representation of the surface to be examined more lucid, the different intensity values can be assigned different colors. In this manner, a multicolored visual representation is shown on the display screen, yielding a rapid surface analysis of the surface to be examined. Naturally, the local etch rate or the local termination point of the etching can also be visually represented.
In particular, the invented laserinterferometer is advantageously suited to carry our the invented process. For construction as an imaging laserinterferometer, only a modulator and a data processing system are additionally employed. Said construction is not substantially much more complicated. Preferably the modulator contains an electromechanical deflector system, which contains an electrically modulated crystal.
To direct the laser beam to the surface to be examined, it is advantageously provided with the lightwave guide. The respective deflector unit is steered over the surface to be examined and adjusted. In order to increase the resolution, it is expedient to insert an appropriate lens system in the path of the beam, to focus the beam to the surface to be examined.
A preferred embodiment of the invented interferometer contains a device to increase the signal-to-noise ratio. A modulation device can be provided for this purpose, which modulates the amplitude of the laser beam with a fixed frequency. The frequency corresponding to a narrow band of the modulation frequency is detected. Moreover, electronic devices can be provided, which separate the mixed signals and suppress the noise.
The invented laserinterferometer is preferably provided with a display screen. The display screen permits advantageously the adjustment of the surface to be examined, in such a manner that an overall impression is given of the surface, by illustration during treatment, particularly an etching process. In this way the operation can be observed and examined "on-line". A plotter can be provided as an alternative for or additionally to a display screen.
The invented interferometer is preferably used to determine the etch rate and the termination point in the etching of thin layers. It can be employed particularly advantageously for process control as data on the entire surface to be examined or to be treated can be ascertained.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.